ELECTRICAL METHODS 



557 



that the drops in the resistivity profiles were due to the presence of a 

 sulphide zone at a depth of about 300 feet.* Hence, diamond drilling 

 was recommended and the hole shown in the figure penetrated a body of 

 sulphides with high copper and silver values. 



The use of this general technique may be illustrated by a survey in 

 Inyo County, California. f Previous history showed that lead-zinc sulphide 

 ore occurred in the limestone, adjacent to a limestone-monzonite contact. 

 This contact initially was discovered at four outcrops over a distance of 

 about a mile. Between outcrops the contact was covered with fill material. 

 This type of sulphide is a good electrical conductor and can be located by 

 determining the subsurface zones of better electrical conductivity. Recon- 



600 1200 I80O FT. 



ELECTROOe JEPARATIOn 



Fig. 342. — Variations of apparent resistivity on a traverse 

 across a highly conductive ore body. (Canad, Min. Journal.) 



naissance measurements were made along the indicated line of contact, 

 using an inductive method. This was followed by detailed resistivity 

 measurements to map the zones of better electrical conductivity. Two 

 main zones of higher conductivity were mapped, one near an old tunnel. 

 Later, a cross-cut to the contact encountered ore, consisting chiefly of lead 

 sulphide replacements, and the large oxidized ore bodies adjacent to them, 

 which now have been mined continuously since 1940.| The development 

 work was extended to cut the second conductive zone, which was at a depth 

 of about 300 feet below the surface. This zone proved to be high-grade 

 sulphides, nearly underlying an old oxidized ore body previously mined 

 and abandoned. 



Prospecting for Placer Deposits.§ — This work may be illustrated 



* Based on a depth of penetration of approximately 0.3 the current electrode 

 separation. 



t J. J. Jakosky, "Operating Principles of Inductive Geophysical Processes," A.I.M.E. Geophys- 

 ical Prospecting, 1929. 



t Personal communication from H. E. Olund, U. S. Bureau of Mines, to S. E. Stein, Oct. 4, 

 1948, regarding the American Metals Co. property at Darwin, Calif. 



§ J. J. Jakosky and C. H. Wilson, "Examining a Placer by Geophysical Methods," Engineering 

 and Mining Journal, February, 1934. 



J. J. Jakosky and C. H. Wilson, "Geophysical Studies in Placer and Water Supply Problems," 

 A.I.M.E. Geophysical Prospecting, Tech. Pub. 515, 1933. 



